spec: Define ACString type and use it for labels values

[sgotti]

This is related to discussion in #586.

I tried to put some limits on the freeform string trying to keep all the possible use cases (and only ACIs with very strange labels will be broken by this change) without limiting them to only ASCII characters . For example kubernetes has very big limits on label values and force it to be <= 63 chars (http://kubernetes.io/docs/user-guide/labels/).

About the max number of chars I just choosed 255 as a number but it can be changed.

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spec: Define ACString type and use it for labels values

Having a label value being a freeform string is coumbersome to use and
query since it can contains non printable characters.
Also having it unlimited can bring to other problems.

This patch adds a ACString type that:

• is restricted to all unicode printable characters. Such characters
  include letters, marks, numbers, punctuation, symbols, and the ASCII
  space character, from unicode categories L, M, N, P, S and the ASCII
  space character.
• it MUST be UTF-8 encoded
• it MUST be be 255 characters or less (not bytes).

This patch starts using this new type for labels values.

Some notes:

• There isn't a regexp to match the valid chars. In Go we can use
  unicode.IsPrint or strconv.IsPrint
• This defines that the string MUST be UTF-8 (needed to correctly count
  the number of characters) but this assumption is done everywhere (for
  example json string marshall/unmarshall) when a string is involved. So
  this should probably be defined at the top of the spec?